Fractionation of five technical lignins by selective extraction in green solvents and characterisation of isolated fractions

Lignins from softwood, hardwood, grass and wheat straw were fractionated by selective extraction at ambient temperature using green solvents like acetone/water solutions of 10, 30, 50, 70 and 90% (v/v) acetone and ethyl acetate. A comparison between the isolated fractions and unfractionated lignins was made in terms of extraction yield, lignin solubility factor, molecular weight distribution and functional group composition. Low molecular weight (LMW) lignin fractions with narrow dispersity are obtained by extraction with ethyl acetate and acetone-water solution containing 30% acetone, with yields depending on the type and the functional group content of lignins. A significant amount (56%) of the organosolv hardwood lignin with low molecular weight (Mw = 1868 g/mol) and low dispersity was isolated from ethyl acetate. Insoluble fractions with very high molecular weight (Mw between 10 and 17 kg/mol) are obtained in low yield from acetone-water solutions with 50, 70 and 90% acetone. LMW lignins are in general less condensed and have lower aliphatic hydroxyl content than parent lignins while the HMW fractions have a higher content of condensed hydroxyls. Principal component analysis on the chemical composition of lignins and isolated fractions determined from 31P NMR data showed the high heterogeneity of the technical lignins. Partial least squares models based on FT-IR spectral data were developed to predict the functional group content determined by quantitative 31P NMR analysis of technical lignins and lignin fractions. This approach can be used to develop simple, rapid and accurate analytical tools to monitor and control the selective fractionation of lignin.